linux-kernel - Re: [PATCH] x86/entry: use STB_GLOBAL for register restoring thunk

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Message-ID: <20201224045502.zkm34cc5srdgpddb@treble>
Date:   Wed, 23 Dec 2020 22:55:02 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Fangrui Song <maskray@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86/entry: use STB_GLOBAL for register restoring thunk

On Wed, Dec 23, 2020 at 03:21:26PM -0800, Nick Desaulniers wrote:
> Arnd found a randconfig that produces the warning:
> 
> arch/x86/entry/thunk_64.o: warning: objtool: missing symbol for insn at
> offset 0x3e
> 
> when building with LLVM_IAS=1 (use Clang's integrated assembler). Josh
> notes:
> 
>   With the LLVM assembler stripping the .text section symbol, objtool
>   has no way to reference this code when it generates ORC unwinder
>   entries, because this code is outside of any ELF function.
> 
> This behavior was implemented as an optimization in LLVM 5 years ago,
> but it's not the first time this has caused issues for objtool.  A patch
> has been authored against LLVM to revert the behavior, which may or may
> not be accepted.  Until then use a global symbol for the thunk that way
> objtool can generate proper unwind info here with LLVM_IAS=1.

As Fangrui pointed out, the section symbol stripping is useful for when
there are a ton of sections like '-ffunction-sections' and
'-fdata-sections'.  Maybe add that justification to the patch
description.

We can try to support it, though I suspect other tools may also end up
getting surprised.

> Cc: Fangrui Song <maskray@...gle.com>
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1209
> Link: https://reviews.llvm.org/D93783
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Code looks familiar ;-)

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh